1. Field of the Invention
The present invention relates to an antenna device, and more particularly, to a multi-band antenna device and radio communication equipment using the antenna device.
2. Description of the Related Art
Recently, portable telephones often include a dual band system using two frequency bands, e.g., those of 800 to 900 MHz and 1800 to 1900 MHz. Reverse F-shaped antennas for receiving and transmitting two frequency bands from a single antenna have been proposed. For example, Japanese Unexamined Patent Application Publication No. Hei, 10-93332 discloses an antenna has resonance frequencies of 1500 MHz and 1900 MHz.
As shown in FIG. 15, this antenna includes a slit 2 provided in a conductor plate 1 to define two radiation conductor plates 3 and 4 having different widths and lengths. A portion of the conductor plate 1 is bent to form a connection conductor plate 5. The radiation conductor plates 3 and 4 are supported on a ground conductor plate 6 by the connection conductor plate 5. High frequency power is supplied to the radiation conductor plates 3 and 4 via a feeding pin 7.
Moreover, the U.S. Pat. Nos. 6,271,794, 6,307,512 and 6,333,716 disclose an antenna in which two metallic patterns having different electrical lengths are provided on the surface of a case for a telephone to produce two radiation elements, such that the antenna has resonance frequencies of 900 MHz and 1800 MHz. This antenna includes a slit provided between the two metallic patterns to enable adjustment of the bandwidths of the resonance frequencies.
According to the examples of the prior art, each antenna is a dual band antenna having two resonance frequencies in frequency bands separated from each other, but has a single resonance characteristic in each frequency band. Accordingly, the size of the antenna must be increased to ensure a necessary bandwidth for each resonance frequency. Thus the size of the antenna cannot be reduced. Moreover, when frequency bands having a single resonance are provided, respectively, the resonance characteristics have a single peak. Thus, a wide frequency band cannot be achieved.